This application claims the priority of Japanese patent documents No. 8-146981, filed Jun. 10, 1996; No. 8-153718, filed Jun. 14, 1996; and No. 8-209587, filed Aug. 8, 1996, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a purification apparatus for an exhaust gas which is discharged or emitted from an internal combustion engine such as an automobile, and particularly to an apparatus which includes a catalyst for purifying an exhaust gas from an internal combustion engine that is operated under a lean air-fuel ratio (a lean burn), and from an automobile which has such a lean burn internal combustion engine.
Exhaust gas discharged from an internal combustion engine such an automobile includes carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) etc. which pollute the environment, adversely affect the human body, and disturb the growth and the development of plants.
Accordingly, up to now a great deal of effort has gone into reducing the amount of such pollutants by improving a combustion in the internal combustion engine, and developing a method for purifying the discharged or the emitted exhaust gas using a catalyst to obtain a steady result.
Gasoline engine vehicles frequently utilize a three component catalyst in which platinum (Pt) and rhodium (Rh) are main active components. The oxidation of HC and CO and the reduction of NOx are carried out at the same time to convert the above air pollution materials to harmless materials.
It is characteristic of a three component catalyst, that it operates effectively only for exhaust gases which are generated within a range (xe2x80x9cwindowxe2x80x9d) in the vicinity of a stoichiometric air-fuel ratio.
In the conventional technique, the air-fuel ratio fluctuates in accordance with an operation condition of the automobile. A fluctuation region is principally controlled to the vicinity of the stoichiometric air-fuel ratio, which is a ratio between A (weight of air) and F (weight of fuel), being about 14.7 in case of the gasoline. Hereinafter, in the present specification, the stoichiometric air-fuel ratio is represented by A/F=14.7, but this value varies in accordance with kinds of the fuels.
However, when the engine is operated under a lean air-fuel ratio in comparison with the stoichiometric air-fuel ratio atmosphere, the fuel consumption can be improved. Therefore, the development of a lean burn combustion technique is promoted; and recently automobiles have been developed in which the engine is combusted under the lean area having the air-fuel ratio of more than 18.
However, when a conventional three component catalyst is adopted for purification of a lean burn exhaust gas, although the oxidation purification with respect to HC and CO is performed effectively, the reduction of NOx is not.
Accordingly, to promote the application of the lean burn system for large size vehicles and to enlarge the lean burn combustion time (that is, enlarge the operation area of the lean burn system), it is necessary to develop an exhaust gas purification technique which is suitable to the lean burn system. Thus, the development of a technique for purifying HC, CO and particularly NOx where a large quantity of oxygen (O2) is included in the exhaust gas, has been promoted vigorously.
Japanese patent laid-open publication No. 61,706/1988 discloses a technique in which HC is supplied upstream of a lean burn exhaust gas. The operation of a catalyst is facilitated by lowering the oxygen (O2) concentration in the exhaust gas to a concentration area for effective functioning of the catalyst.
Japanese patent laid-open publication No. 97,630/1987, Japanese patent laid-open publication No. 106,826/1987 and Japanese patent laid-open publication No. 117,620/1987, propose a technique in which N included in the exhaust gas (after the conversion of an easily absorbable N2, by oxidizing NO) is absorbed and removed by contact with a catalyst having NOx absorbing ability. When the absorption efficiency decreases, by stopping a passing-through of the exhaust gas accumulated NOx is reduction-removed using H2, HC included in a methane gas and a gasoline etc., and so that NOx absorbing ability of the catalyst is regenerated.
Further, WO 93/07363 and WO 93/08383 discloses an exhaust gas purification apparatus in which an NOx absorbent material arranged at an exhaust gas flow passage absorbs NOx from a lean exhaust gas, and when an oxygen concentration in the exhaust gas is lowered the NOx absorbent material discharges the absorbed NOx. The exhaust gas absorbs NOx during the lean atmosphere and the absorbed NOx is discharged by lowering O2, concentration in the exhaust gas which flows into a NOx absorbent.
However, in Japanese patent laid-open publication No. 61,708/1988, to attain a composition of the exhaust gas which corresponds to the air-fuel ratio of A/F=14.7 where the catalyst can function (O2 concentration having about 0.5%), it needs a very large quantity of HC. A use of a blow-by gas in this patent document is effective, but the blow-by gas does not have an amount which is sufficient for efficient to treat an exhaust gas during an operation of an internal combustion engine. It is possible technically to throw the fuel but it eliminates the fuel consumption gains achieved by the lean burn system.
In Japanese patent laid-open publication No. 97,630/1987, Japanese patent laid-open publication No. 106,826/1987 and Japanese patent laid-open publication No. 117,620/1987, to regenerate a NOx absorbent material a flow of exhaust gas is stopped and the gaseous reducing agent of HC etc. is contacted to NOx absorbent. Further, two NOx absorbent materials are provided and the exhaust gas flows alternately to these two NOx absorbent materials. It is therefore necessary to provide an exhaust gas change-over mechanism, which complicates the structure of the exhaust gas treatment apparatus.
In WO 93/07363 and WO 93/08383, the exhaust gas is flowed continuously to an NOx absorbent material, and the NOx in the exhaust gas is absorbed during the lean atmosphere. By lowering O2 concentration in the exhaust gas, the absorbed NOx is discharged and the NOx absorbent material is regenerated. Accordingly, since the change-over of the exhaust gas flow is unnecessary, the problem in the above stated system can dissolved. However, these systems require a material which can absorb NOx during the lean condition and can discharge NOx when O2 concentration in the exhaust gas is lowered. Since the repeated NOx absorption and discharge inevitably causes a periodic change of a crystal structure of the absorbent, it is necessary to take a careful consideration about the durability of the absorbent. Further, it is necessary to treat the discharged NOx; in the case of a large quantity of the discharged NOx it may be necessary to provide a post-treatment using a three component catalyst.
In the light of the problems in the above stated prior arts, the object of the present invention is to provide an internal combustion engine exhaust gas purification apparatus which has a simple structure, consumes a small amount of gaseous reducing agents, has superior endurance and which effectively removes harmful components such as NOx from a lean burn exhaust gas converting them to a harmless component.
Another object of the present invention is to provide a catalyst for use in an exhaust gas purification apparatus of an internal combustion engine.
According to the present invention, an exhaust gas purification apparatus for use in an internal combustion engine comprises an exhaust gas duct connected to the engine, through which the exhaust gas containing NOx gas passes, and a catalyst disposed in the exhaust gas duct so that it contacts the exhaust gas. The catalyst chemically adsorbs NOx when a stoichiometric amount of a gaseous oxidizing agent present in the exhaust gas is larger than the amount of a gaseous reducing agent present in the exhaust gas for reducing NOx, while adsorbed NOx is catalytically reduced in the presence of the reducing agent when the stoichiometric amount of the oxidizing agent is not larger that of the reducing agent.
According to the present invention, an apparatus for purifying an exhaust gas from an internal combustion engine comprises an exhaust gas duct connected to the engine, through which the exhaust gas containing NOx gas passes, and a catalyst disposed in the exhaust gas duct so that it contacts the exhaust gas. The catalyst adsorbs NOx under when the amount of a gaseous oxidizing agent present in the exhaust gas is larger than that of a gaseous reducing agent for NOx added to the exhaust gas in a stoichiometric relation, while adsorbed NOx is catalytically reduced in the presence of the reducing agent when the amount of the oxidizing agent is not larger that of the reducing agent in the stoichiometric relation.
According to the present invention, an apparatus for purifying an exhaust gas from an internal combustion engine comprises an exhaust gas duct connected to the engine, through which the exhaust gas containing NOx gas passes, and a catalyst disposed in the exhaust gas duct so that it contacts with the exhaust gas. The catalyst adsorbs NOx when a stoichiometric amount of a gaseous oxidizing agent present in a lean combustion exhaust gas is larger than that of a gaseous reducing agent present in the lean combustion exhaust gas for reducing NOx, while the adsorbed NOx is catalytically reduced in the presence of the reducing agent when a stoichiometric amount of the oxidizing agent is not larger that of the reducing agent in a stoichiometric or fuel rich combustion exhaust gas.
According to the present invention, an apparatus for purifying an exhaust gas from an internal combustion engine comprises an exhaust gas duct connected to the engine, through which the exhaust gas containing NOx gas passes, a device for controlling the air-fuel ratio of the exhaust gas, and a catalyst disposed in the exhaust gas duct so that it contacts with the exhaust gas. The air-fuel ratio is switched from a range in which the catalyst chemically adsorbs NOx in the lean combustion exhaust gas to a range in which adsorbed NOx is catalytically reduced in the presence of the reducing agent in a stoichiometric or fuel rich combustion exhaust gas.
The catalyst according to the present invention comprises a heat resistant carrier body and catalytic compounds supported thereon. The catalytic compounds comprise at least one of sodium and potassium, at least one of magnesium, strontium and calcium, and at least one of platinum, palladium and rhodium. In at least some embodiments, a base member supports the heat resistant carrier body.
According to the present invention, the reducing agent to be added to the lean combustion exhaust gas is at least one of gasoline, light oil, kerosene, natural gas, their reformed substances, hydrogen, alcohol, ammonia gas, engine blow-by gas and canister purging gas. The reducing agent is supplied to the lean combustion exhaust gas in response to signals from the stoichiometric establishing unit.
The exhaust gas purification apparatus according to the present invention further comprises a manifold catalyst which is disposed in the exhaust gas duct immediately after the engine, upstream of the catalyst, and functions as a three component catalyst and a combustion catalyst.
According to another feature of the present invention, in the exhaust gas purification apparatus, a second catalyst is disposed in the exhaust gas duct of a direct fuel injection engine.
According to yet another feature of the present invention, the exhaust gas purification apparatus further comprises a three component catalyst or a combustion catalyst is disposed in the exhaust gas duct upstream of the catalyst.
According to the present invention, an apparatus for purifying an exhaust gas from an internal combustion engine comprises an exhaust gas duct connected to the engine, through which the exhaust gas containing NOx, SOx and oxygen passes, and a catalyst which chemically adsorbs NOx when the exhaust gas is emitted from lean combustion, and in which adsorbed NOx is catalytically reduced when a gaseous reducing agent is added to the lean combustion exhaust gas in such an amount that a stoichiometric amount of oxygen is not larger than that of the reducing agent.
The catalyst adsorbs or absorbs SOx in the lean condition, and releases SOX in the stoichiometric or rich condition.
According to the present invention, a catalyst for purifying an exhaust gas from an internal combustion engine comprises a base member, a heat resistant carrier body supported on the base member, and catalyst components supported on the carrier body. The carrier body has a number of small hollows extending in the direction of gas flow of the exhaust gas.
According to the present invention, the catalyst compounds comprise at least one alkali metal, at least one alkali earth metal (other than barium), at least one noble metal and at least one rare earth metal.
According to the present invention, an exhaust gas purification apparatus for use in an internal combustion engine comprises a catalyst which chemically adsorbs NOx when the amount of a gaseous oxidizing agent is greater than that of a gaseous reducing agent in a stoichiometric relation between the gaseous oxidizing agent and the gaseous reducing agent, and catalytically reduces adsorbed NOx when the gaseous reducing agent is equal to or exceeds the gaseous oxidizing agent. The catalyst is provided in an exhaust gas flow passage for a flow of an exhaust gas generated at a lean air-fuel ratio and at a rich air-fuel ratio or a stoichiometric air-fuel ratio.
According to the present invention, an exhaust gas purification apparatus for use in an internal combustion engine comprising a catalyst for chemically adsorbing NOx under a condition where an gaseous oxidizing agent is more than a gaseous reducing agent in a stoichiometric relation between the gaseous oxidizing agent and the gaseous reducing agent and a device which controls an air-fuel ratio to switch conditions between NOx chemical adsorption to the catalyst and catalytic reduction chemically of NOx to the catalyst.
According to the present invention, an exhaust gas purification apparatus comprises a catalyst which chemically adsorbs NOx when, in a stoichiometric relation between gaseous oxidizing agent and gaseous reducing agent in an exhaust gas flowing an exhaust gas flow passage in the internal combustion engine, the amount of gaseous oxidizing agent is more than the amount of gaseous reducing agent, and catalytic-reduces adsorbed NOx when the amount of gaseous oxidizing agent equals or exceeds the amount of gaseous reducing agent. The catalyst is provided in the exhaust gas flow passage where an exhaust gas burned at a lean air-fuel ratio and an exhaust gas burned at a rich or stoichiometric air-fuel ratio flow into alternately.
The exhaust gas purification apparatus according to the present invention provides a stoichiometric relation between oxidation and reduction of gaseous oxidizing agent and gaseous reducing agent. A control means for controlling the stoichiometric relation between gaseous oxidizing agents and gaseous reducing agents comprises a timing control means for controlling the time at which the stoichiometric relation between gaseous oxidizing agent and gaseous reducing agent changes over from a first condition in which the amount of gaseous oxidizing agent is more than that of a gaseous reducing agent, to another condition in which the amount of gaseous reducing agent is equal to or exceeds the amount of gaseous oxidizing agent. It also comprises a gaseous reducing agent excess time control means for controlling a time when, in the stoichiometric relation between oxidation and reduction, the gaseous reducing agent is held to an amount equal to or greater than the gaseous oxidizing agent.
According to the present invention, the catalyst has the ability to chemical adsorb NOx, and to catalytic-reduce NOx. Even when the oxygen concentration decreases, the catalyst does not discharge NOx. These features can be obtained by a catalyst which comprises at least one element selected from among the alkali metals and alkali earth metals in the element periodic table (but not including barium (Ba)), and at least one selected from noble metals comprising platinum (Pt), palladium (Pd) and rhodium (Rh). Further, the catalyst has an ability for catalytic oxidizing HC or CO etc.
The catalyst has a high NOx absorption ability in a lean atmosphere and further under the catalyst temperature of 250-500xc2x0 C. Further, to recover its NOx adsorbing ability by reduction of the absorbed NOx, the catalyst must maintain a stoichiometric atmosphere or a rich atmosphere for a time of about 30 seconds or less.
Accordingly, to perform effectively NOx adsorption and to recover NOx adsorbing ability, it is desirable to provide the catalyst at a position of an exhaust gas duct where an inlet port gas temperature of the catalyst is 250-500xc2x0 C. The above temperature range is one which can obtain normally under the car-body floor.
NOx adsorbing ability of the catalyst is lowered due to poisoning by SOx originated fuel (gasoline). However, when the catalyst is maintained several (for example, ten) minutes at 400-800xc2x0 C. in a stoichiometric or rich atmosphere, SOx is removed and then NOx adsorbing ability is recovered.
Accordingly, when the gasoline quality is bad (high sulfur content) and the catalyst suffers from poisoning by SOx, it is desirable to position the catalyst in an exhaust gas duct where an inlet port gas temperature of the catalyst is 400-800xc2x0 C. The above temperature range is one which can obtain under the car-body floor.
When the catalyst is used for an automobile, it is desirable to form it as a honeycomb having NOx adsorption ability of more than 0.01 mol per an apparent honeycomb volume one (1) liter.
Further, it is desirable to set the specific surface area of the catalyst. layer on the honeycomb substrate (the honeycomb base body), measured by absorbing nitrogen according to BET method, at more than 50 m2/g.
In the exhaust gas, the gaseous oxidizing agents are O2, NO, NO2, etc. and are mainly oxygen. The gaseous reducing agents are HC supplied in an internal combustion engine, HC (including oxygen containing hydrocarbon) generated in a combusting process as a derivative from fuel, CO, H2 etc. Furthermore, a reducing material such as HC can be added in the exhaust gas as a reducing component.
When the lean exhaust gas contacts the three component catalyst, HC, CO, H2 etc. as the gaseous reducing agents for reducing NOx to nitrogen (N2), cause a combustion reaction with oxygen (O2) as the gaseous oxidizing agent in the exhaust gas. NOx (NO and NO2) reacts with these gaseous reducing agents and is reduced to nitrogen (N2). Normally, since both reactions proceed in parallel, a utilization rate of the gaseous reducing agents for reducing NOx is low.
Particularly at a high reaction temperature of more than 500xc2x0 C. (depending on the catalyst material), an occupation rate of the latter reaction becomes large. Hence, by separating NOx from the exhaust gas (at least from O2) using the catalyst, and then carrying out the catalytic-reaction with the gaseous reducing agents, it is possible to achieve an effective reduction of NOx to N2. According to the present invention, the catalyst is used to adsorb and remove NOx in the lean exhaust gas, whereby NOx in the exhaust gas is separated from O2.
Next, according to the present invention, with regard to the oxidation reduction relation, namely the stoichiometric relation between oxidation and reduction, which is constituted by the gaseous oxidizing agents (O2 and NOx etc.) and the gaseous reducing agents (HC, CO, H2 etc.) in the exhaust gas, the gaseous reducing agent is made equal to or larger than the gaseous oxidizing agent. In this manner NOx adsorbed on the catalyst is reduced to N2 according to the catalytic-reaction with the gaseous reducing agent such as HC.
NOx in the exhaust gas is substantially constituted of NO and NO2. The reaction property of NO2 is rich in comparison with that of NO. Accordingly, when NO is oxidized to NO2, the adsorption-removal and the reduction of NOx in the exhaust gas are performed easily.
The present invention includes a method of oxidizing and removing NOx in the exhaust gas to NO2 by the coexistent O2, and an oxidation means for attaining the above method (such as means having NO oxidation function and the means for providing an oxidation catalyst at a pre-stage of the catalyst).
The reduction reaction of a chemically adsorbed NO2 according to the present invention will be described generally with following reaction formulas:
MOxe2x88x92NO2+HCxe2x86x92MO+N2+CO2+H2Oxe2x86x92MCO3+N2+H2O
where M indicates a metal element and MOxe2x88x92NO2 indicates a combination State Of NO2 of a metal oxide surface. A reason for employing MCO3 as the reduction generation substance will be explained later.
The above reaction is exothermic. Alkali metals and alkali earth metals are used as the metal M, and the reaction heat is estimated by representing Na and Ba respectively as follows, under a standard condition (1 atmosphere, 25xc2x0 C.):
2NaNO3 (s)+5/9C3 H6 (g)xe2x86x922Na2NO3 (S)+N2 (g)+2/3CO2(g)+5/3CO2 (g) [xe2x88x92xcex94H=873 k joule]
Ba(NO3)2 (s)+5/9C3H6, (g)xe2x86x92BaCO3 (s)+N2 (g)+2/3CO2 (g)+5/3H2O (g) [xe2x88x92xcex94H=751 k joule]
wherein, s indicates a solid state and g indicates a gaseous state. Here, a thermodynamic value of the solid state is used, as that of an adsorbed state.
It should be noted that the combustion heat of 5/9 mole C3H6 is 1,070 k joule, and each of the above reactions is exothermic. The heat, which matches that for the combustion beat of HC, heat is transferred to the exhaust gas, and thus a local rise in temperature of a surface of the catalyst can be restrained.
Where the catching agent for NOx is an NOx absorbent, NOx which is taken into the mass of the absorbent is reduced. Heat transfer to the exhaust gas is limited, causing a rise in temperature of the absorbent. This exothermically generated heat shifts the balance of the absorption reaction to that of NOx discharging or NOx emission.
absorption MCO3 (s)+2NO2+1/2O2←xe2x86x92M(NO3)2+CO2←discharging
Even though the concentration of the gaseous reducing agents is increased to reduce rapidly NOx concentration in the exhaust gas which is discharged to the outside of the absorbent, the reaction between NO2 and HC in the gaseous phase does not proceed.
Accordingly, the amount of discharged NOx cannot be reduced fully by an increment of the gaseous reducing agents. Further, at a stage where the adsorption amount of NOx is small, a reduction operation may occur; however, since the regeneration frequency of NOx absorbent increases, it is not put to practical use.
The catalyst according to the present invention generates a small absolute amount of exothermic heat so as to catch NOx near its surface in accordance with chemical adsorption. Also the rise in temperature of the catalyst is small, so as to transfer heat rapidly to the exhaust gas. Accordingly, it is possible to prevent the discharge of NOx after it is captured.
The catalyst according to the present invention utilizes a material which catches NOx at or near its surface by chemical adsorption, and does not cause NOx discharging or NOx emission in accordance with the exothermic reaction during the reduction of NOx.
Further, the catalyst according to the present invention adsorbs NOx contained in lean exhaust gas at its surface, and during the reduction of NOx it does not cause NOx discharge in accordance with the lowering of the oxygen concentration.
The inventors of the present invention have determined that the above stated features can be realized using a catalyst which is selected from at least one of alkali metals and alkali earth metals (classified in an element periodic table) and at least one of noble metals selected from platinum (Pt), rhodium (Rh) and palladium (Pd), but does not contain barium (Ba). Preferably, the catalyst according to the present invention includes at least one element selected from potassium (K), sodium (Na), and strontium (Sr), and noble metal elements.
In the exhaust gas purification apparatus according to the present invention, a catalyst arranged at an exhaust gas flow passage includes at least one element selected from potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca), as well as noble metal elements.
In the exhaust gas purification apparatus, in the case of a stoichiometric relation between oxidation and reduction of each of the components included in the exhaust gas, the gaseous oxidizing agent is equal to or greater than the gaseous reducing agent, and NOx is chemically adsorbed on the catalyst. On the other hand, when the gaseous reducing agent is equal to or greater than the gaseous oxidizing agent, NOx which has been absorbed on the catalyst is reduced, according to the catalytic-reaction with the gaseous reducing agent, to harmless N2.
The catalyst in the present invention can be applied suitably in particular by following substances.
The composition is constituted of a metal and a metal oxide substance (or a complex oxide substance) which contains at least one element selected from potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca), at least one selected from rare earth metals, and at least one element selected from the noble metals including platinum (Pt), rhodium, (Rh), and palladium (Pd). This composition, which is supported on a porous heat-withstanding metal oxide substance, has a superior NOx adsorbing ability.
As the earth metal element, cerium (Ce) or lanthanum (La), particularly Ce, is preferable. The earth metal element has a function for exhibiting the three component function to the catalyst under the stoichiometric atmosphere or the rich atmosphere.
At least one of titanium (Ti) and silicon (Si) can be added to the catalyst according to the present invention, improving the heat resistant property and SOx endurance property of the catalyst. Ti or Si has a function for adsorbing or absorbing SOx under the lean atmosphere, or for discharging the adsorbed or absorbed SOx in a stoichiometric atmosphere or a rich atmosphere.
In the catalyst of the present invention, alkali metals, alkali earth metals, noble metals, rare earth elements, titanium (Ti) and silicon (Si) are held on the porous support or porous carrier member, which is supported or carried on a substance body. For the purpose of heat resistance, xcex3-Al2O1 is preferably employed as the porous support. As the substance body, a cordierite, mullite, a metal, for example, a stainless steel is preferable.
As the crystal structure of Ti which is held on the porous support, an amorphous oxide state is preferable. Further, in case where the catalyst includes Si and alkali earth metals at the same time, as both the crystal structures Si and alkali earth metals, an amorphous oxide state is preferable.
In the catalyst of the present invention, it is preferable to include in the porous support (porous carrier member), alkali metals of 5-20 wt %, and alkali earth metals of 3-40 wt %. Further, it is also preferable to include Pt of 0.5-3 wt %, Rh of 0.05-0.3 wt %, and Pd of 0.5-15 wt %, respectively. Mg prevents the cohesion or condensation of the active components which are held on the porous support, such as the noble metal.
It is also preferable to include the rare earth metals of 5-30 wt %, Ti of 0.1-30 wt %, and Si of 0.6-5 wt % as silica in the porous support.
The present invention provides a catalyst which comprises, on the porous support, sodium (Na), magnesium (Mg), and at least one element selected from platinum (Pt), palladium (Pd), and rhodium (Rh), as well as at least one selected from cerium (Ce) and lanthanum (La). Further, the porous support also preferably includes Na of 5-20 wt %, Mg of 1-40 wt % under a weight ratio Mg/(Na+Mg), Pt of 0.5-3 wt %, Rh of 0.05-0.3 wt %, and Pd of 0.5-15 wt % are included.
In the exhaust gas purification apparatus according to the present invention, to chemically adsorb NOx to the catalyst, or to catalytically reduce the chemically adsorbed NOx, means must be provided for controlling the stoichiometric relation between oxidation and reduction by the gaseous oxidizing agents and the gaseous reducing agents in the exhaust gas. By providing a means for controlling the stoichiometric relation between oxidation and reduction, it is possible to assure that the gaseous reducing agent equals or exceeds the gaseous oxidizing agent. For example, the combustion condition in the internal combustion engine can be adjusted to a stoichiometric or a rich air-fuel ratio, or a gaseous reducing agent can be added to the lean burn exhaust gas.
One method of achieving the former is to control the fuel injection amount in accordance with the output of the oxygen concentration sensor and the output of the intake air flow amount sensor provided in the exhaust gas duct. In this method, some of the cylinders are operated with a rich mixture and the remainder are operated in a lean mixture. In the mixed components of the exhaust gas all of the cylinders the gaseous reducing agent is equal to or is greater than the gaseous oxidizing agent in the stoichiometric relation between the oxidation and the reduction.
The latter can be attained by each of following methods.
One method is to add a gaseous reducing agent to the exhaust gas flow upstream of the catalyst. As the gaseous reducing agent, gasoline, light gas oil, natural gas, reforming material thoseof, hydrogen, alcohol materials and ammonium materials can be applied. It is effective to introduce the blow-by gas and the canister purging gas at the upstream of the catalyst and to add the gaseous reducing agent such as hydrocarbon (HC) contained in the above materials. In an internal combustion engine with direct fuel injection, it is effective to inject the fuel during the exhausting process and to add the fuel as the gaseous reducing agent.
As the catalyst in the present invention, various shapes can be applied. In addition to a honeycomb shape which is obtained by coating the catalyst components onto a honeycomb shaped member comprised of cordierite or metal materials such as a stainless steel, a pellet shape, a plate shape, a particle shape and a powder shape can be applied.
In the present invention, the apparatus can provide means for establishing the time when the gaseous reducing agent is equal to or is greater than the gaseous oxidizing agent. The above timing is obtained by each of following methods.
In one technique, in accordance with the air-fuel ratio setting signal which is determined by ECU (engine control unit), the engine rotation number signal, the intake air amount signal, the intake air pipe pressure signal, the speed signal, the throttle valve opening degree signal, the exhaust temperature etc., the NOx discharging amount during lean operation is estimated and the integration value thereof is exceeds over a predetermined setting value.
In another method, in accordance with the signal of the oxygen sensor (or A/F sensor) arranged upstream or downstream of the catalyst in the exhaust gas flow passage, the accumulated oxygen amount is detected, and the accumulated oxygen amount exceeds over a predetermined amount. As a modified embodiment, the accumulated oxygen amount during the lean operation time exceeds a predetermined amount.
In another approach, in accordance with the signal of NOx sensor arranged at the upstream of the catalyst in the exhaust gas flow passage, the accumulated NOx amount is detected, and the accumulated NOx amount during the lean operation time exceeds over a predetermined amount.
In still another method, in accordance with the signal of NOx sensor arranged at the downstream of the catalyst in the exhaust gas flow passage, NOx concentration is detected, and NOx concentration exceeds over a predetermined concentration.
According to the present invention, further the apparatus provides the means for establishing the maintenance time where the gaseous reducing agent is equal to or exceeds the gaseous oxidizing agent. The time during which the gaseous reducing agent excess condition and the throw-in gaseous reducing agent amount is maintained can be determined taking into consideration the specifications and characteristics of the adsorbent and the internal combustion engine. The above methods can be realized by adjusting the stroke, the injection time and the injection interval of the fuel injector.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.